In general, audio reproduction systems are designed to produce a specific amount of audio power while specifically limiting the amount of audio distortion. Conventional audio reproduction systems include an audio source, such as an AM/FM tuner, a cassette deck, a CD player, or the like, that feeds a digital audio signal to a digital signal processor (DSP). The DSP includes a variable gain amplification stage for processing the audio signal and feeding an output signal through a fixed-gain power amplifier to a speaker or plurality of speakers. The DSP receives operator inputs such as volume and tone control (e.g. bass boost, treble and the like) for varying the amplification of respective frequencies during the variable gain amplification stage.
Audio distortion can occur when an operator increases the volume amplification and/or boosts any of the tone controls. The amplified audio signal (i.e. power amplifier output signal) may grow to the point where its amplitude approaches the power supply voltage limits. Typical power amplifiers include clipping circuits for “clipping” the signal peaks as they reach the power supply limit. Clipping of the audio signal is undesirable in that it causes new audio components to be introduced into the original signal. The new audio components occur at odd harmonics of the frequency associated with the clipped signal, thus distorting the original audio signal.
Prior art methods for controlling distortion include: implementation of voltage limiting or compression to the input of a power amplifier to prevent clipping, comparing the power output signal to a predetermined reference and attenuating the signal when the output signal exceeds the predetermined reference, or separating the audio signal into separate paths for bass frequencies and higher frequencies, then reducing the gain of each path upon the occurrence of a peak level. Each of these methods, however, includes associated disadvantages that inhibit cost efficient implementation.
Another method for controlling distortion provides a micro-controller for managing the operation of an audio system having a DSP that controls a wideband and a narrowband gain for the audio signal. Clipping is avoided by sensing clipping in the power amplifier and initially reducing the narrowband gain and subsequently reducing the wideband gain if clipping persists. Upon cessation of clipping, the narrowband gain is initially restored and the wideband gain is subsequently restored if clipping is still not sensed. This method, however, includes certain disadvantages including a fixed value of reduction in each of the gains and the order of restoring the previously reduced gains.